Dry nuclear spent fuel storage technology is deployed throughout the world to expand the capabilities of nuclear power plants to discharge and store spent fuel, thereby extending the operating lives of the power plants. Two fundamental classes of technology are used in dry spent fuel storage: metal casks with final closure lids that are bolted closed at the power plants after loading with spent fuel, and concrete storage casks containing metal canisters having canister final closure lids that are welded closed at the power plants following spent fuel loading. This latter technology is referred to as canister-based dry spent fuel storage technology.
Canister-based dry spent fuel storage technology designs typically comply with federal regulatory requirements, among which are requirements pertaining to confinement, sealing, inspection, and evaluation or testing. In particular, the Code of Federal Regulations (CFR) requires that “[t]he spent fuel storage cask must be designed to provide redundant sealing of confinement systems.” See 10CFR72.236(e). The confinement system is defined in 10CFR72.3, as follows: “Confinement systems means those systems, including ventilation, that act as barriers between areas containing radioactive substances and the environment.” In other words, the confinement system is the boundary that is to be protected to assure that there is no release to the environment of the contained radioactive materials within the spent fuel dry storage system.
The Nuclear Regulatory Commission (NRC), which enforces compliance with federal regulations, has applied the above requirements to fabrication facility-installed, confinement system welds in canister-based dry storage designs (those which are performed prior to delivery to the power plant and prior to loading with spent fuel) such that single or multi-pass welds that are inspected with NRC approved inspection methods and techniques meet the requirements of 10CFR72.236(e). However, for final closure welds on canister-based technology, which are conducted at nuclear facilities following loading of spent fuel, the application of the regulatory requirements has varied, but NRC approval of the varied approaches has been consistent with the use of approved single or multi-pass welding approaches followed by the application of approved inspection methods and techniques.
Over the years, most canister-based dry storage designs have followed a path to assure compliance with redundant sealing requirements for the final canister closure at the power plant (the “field closure”) by the use of redundant lids for the final closure design. That is, two lids or closures were placed within the canister and welded to the canister shell. These lids were not required to be structurally separate (independent) and could be linked to each other by additional welding. However, approaches with and without structural independence of the redundant lids have all been approved by the NRC for compliance with federal requirements.
The use of a redundant lid approach on canister-based designs leads to lid handling and installation complexities at power plants. With multiple lid handling operations, opportunities for operator injury and equipment damage are increased, and more time must be spent in higher radiation fields. Further, the use of redundant lids increases the radiation exposure of power plant personnel when compared to a single lid approach, since more time is required to handle, lift, install and weld two lids, because the first lid to be welded of a redundant lid design provides less radiation shielding to the operators, and, therefore, more radiation exposure. Such an approach may be at variance with federal regulatory requirements, if there is a reasonable alternative that reduces direct radiation levels, to wit: “Operational restrictions must be established to meet as low as reasonably achievable [known as ALARA in industry parlance, a design objective to keep operator radiation exposures reasonably low] objectives for radioactive materials in effluents and direct radiation levels associated with ISFSI or MRS operations.” (See 10CFR72.104(b).)
Thus, a heretofore unaddressed need exists in the industry to improve upon the aforementioned deficiencies and inadequacies.